Range imaging systems based on indirect time-of-flight measurement techniques determine both distance and intensity simultaneously for every pixel in an image. Typically, a scene is illuminated with an intensity modulated light source and the reflected signal is sampled using a gain-modulated (or shuttered) imager. The frequencies used to drive the gain modulation (also called the high-speed shutter) and light source are frequently between 10 MHz and 100 MHz. These systems are capable of effectively measuring the size, shape and location of objects in a scene thereby enabling many applications in areas such as automotive safety and control, multimedia user interface, image segmentation and surface profiling, for example. The parallel nature of the range acquisition allows depth images to be acquired at video frame rates, yet systems can have the flexibility to operate in slower modes to achieve sub-millimeter depth resolution.
A typical Amplitude Modulated Continuous Wave (“AMCW”) range imaging system employs, among other elements, a signal source, an illumination system, a sensor, and a processor. The signal source emits two signals, one of which modulates the scene illumination that is reflected by the object/s. The sensor detects the reflected light using the second signal and the processor then calculates the object's distance and/or intensity by analyzing the detected signal.
Methods for analyzing signal measurements are disclosed in, for example, U.S. Pat. No. 7,561,255 to Billmers et al., U.S. Pat. No. 5,835,199 to Phillips et al., and U.S. Pat. No. 5,534,993 to Ball et al., all of which are herein incorporated by reference in their entireties. Also, see International Publication No. 2004/090568 to Donington, International Publication No. 2006/031130 to Carnegie et al., and International Publication No. 2009/051499 to Payne et al., all of which are herein incorporated by reference in their entireties. Methods and apparatuses for detecting and ameliorating signal interference are disclosed in, for example, U.S. Pat. No. 7,697,119 to Ikeno et al., which is herein incorporated by reference in its entirety. A method of three dimensional imaging is disclosed in, for example, U.S. patent application Ser. No. 12/819,602 (U.S. Patent Publication No. 2010/0258708) to Meyers, et al., which is herein incorporated by reference in its entirety.
Typical AMCW lidar systems suffer from a problem known as mixed pixels, multipath or crosstalk. Mixed pixels occur when a single range/intensity measurement is composed of multiple objects at more than one distance from the camera. Multiple objects result in multiple reflected signals, causing phase shifts and intensity changes in the measured backscattered signal and, consequently, erroneous range and intensity values.